There exists already for a long time a strong and even increasing demand for photosensitive materials that have improved sensitivity, that is for materials that respond to a decreasing amount of light energy. A very interesting possibility is found in photosensitive materials wherein the primary light-activated change exists on an atomic or molecular level which in a secondary step can be multiplicated by several orders of magnitude in order to visualize the first light interaction in the material. This `two-step` image formation mechanism is for instance encountered in silver halide materials which form the main subject of this invention. It will be clear that sensitivity in this type of materials is determined by the efficiency in which the different steps between the light interaction with the silver halide and the formation of the visual image can proceed. This invention will be specially focused on the way in which the first molecular light-induced change in silver halide crystals is realized. This change on atomic level (also called latent image formation) gives rise to the formation of a visual image after development.
The efficiency of the latent image formation depends on many factors and can therefore be influenced in many different ways. The best result is realized if each photoelectron, created after light absorption in the silver halide crystal reaches the deepest electron trap while forming the latent image. This means that the recombination between holes and electrons that are created after light absorption is as much as possible prevented. Many solutions are proposed but all of them have a limited result. One can primarily try to lower the depth of electron trap in order to increase the capture probability. Chemical sensitization with for instance sulphur, gold, selenium and other compounds or combinations thereof is mostly used for this purpose.
Another way in preventing the recombination of holes and electrons after formation is the temporary interception of these species at local traps with intermediate trap depth. This can be realized by creating a distortion internally in the crystal latice for instance by the local incorporation of an increased amount of iodide in the core or in a certain zone of the grain. Although this method leads to a sensitivity gain by decreasing the electron-hole recombination, another important feature like developability, which is wanted in modern photographic materials is deteriorated by the presence of the iodide. Temporary interceptions of the electron can be realized in many different ways and are described in many patents as for instance in U.S. Pat. No. 2,448,060, EP-A 0 336 425, EP-A 0 336 426, EP-A 0 336 427 and EP-A 0 415 480. In these cases a metal complex dopant was introduced in the silver halide crystals which always resulted in an increased sensitivity. In relation with the present invention special attention is drawn on the electron trapping agents containing CO-ligands. In EP-A 0 415 481 is McDuggle, et al. describing a dopant of the type [M(CO).sub.m L.sub.6-m ].sup.n wherein M is a metal of the 8.sup.th and 9.sup.th group of the Periodic System of Elements and L is ligand (halide or others). While m=1, 2 or 3 it can be concluded that always another ligand than the CO-ligand is present in the dopant. Evans et al. describe in U.S. Pat. No. 5,024,931 a metal complex with general formula M.sub.n X.sub.m where M are metals of the 8.sup.th, 9.sup.th or 10.sup.th group of the Periodic System of Elements and X represents a halide, pseudo-halide or CO, NO, NS, N.sub.3, O or H.sub.2 O. Further is 4.ltoreq.m/n&lt;5 if 2&lt;n.ltoreq.5, m/n=4 if 6.ltoreq.n.ltoreq.8 and m/n&lt;4 if n&gt;8. JP-A 04-125 629 describes an increase in sensitivity of a chloride-rich AgClBr-emulsion after doping with a metal complex consisting of a metal out of the 7.sup.th up to 10.sup.th and the 13th group of the Periodic System of Elements and a ligand which can be a pseudohalide, a carbonyl or another ligand. All the complexes contain one central metal atom except for rhenium. Further JP-A 06-148 784 is describing the doping of a silver halide emulsion with metal-CO-complexes (where the mentioned metals are: Cr, Mo, W and Ru) resulting in an increase of sensitivity.
Increasing the sensitivity of a silver halide emulsion can also be realized by increasing the efficiency of electron transfer from the spectral sensitizer to the silver halide grain which principally can be carried out with a supersensitizer.
Looking at the activity of the sensitizing dye, sensitivity gain can also be realized by decreasing the dye desensitization which is evolved by increasing dye concentration at the grain surface. This can for instance be done by combining an electron donating compound like ascorbic acid with specific cyanine and merocyanine dyes as described in U.S. Pat. No. 4,897,343. An electron-donating compound that is attached to a sensitizing dye or a silver halide absorptive group have also been used to get a additional sensitizing effect as is described in U.S. Pat. No. 5,436,121 and U.S. Pat. No. 5,478,719.
Another interesting alternative to decrease the recombination of the photo-electron and hole is introducing hole traps like silver clusters or some metal complexes in the silver halide crystal. Silver clusters can be created in crystals by reduction sensitization which is realized by treating the emulsion during the precipitation with a reductor like tin compounds, polyamine derivatives, hydrazines, ascorbic acid and analogues, etc. or by well defined pH- and/or pAg-conditions without using reducing substances. This is for instance in U.S. Pat. No. 3,892,574 which describes a method, wherein during precipitation of the silver halide or before or during physical ripening small silver specks (which do not give spontaneously developable fog) are created in reducing conditions. The same can be said for silver halide preparation methods as described in U.S. Pat. No. 3,957,490, where at the end of a reduction periode an oxidant is introduced in the silver halide emulsion before chemical sensitizing. In most of the methods mentioned before it is very difficult to suppress fog formation till an acceptable level which is experienced as a serious problem.
In all the concepts mentioned hereinbefore it is also experienced that these silver clusters can be easily formed on {111}-AgBr crystal faces while it becomes difficult for instance on {100}-AgBr and {100}-AgCl crystal faces. An alternative way for forming silver clusters for this particular application is described in EP-Application Nr. 97/203897.0 (filed Dec. 12, 1997). In this proposal silver halide crystals have to be doped with an organic hole-trapping agent represented by R-COOM as general formula wherein R is H or a substituted or unsubstituted alkyl, aryl or aralkyl, while M is H or any metal or organic group which can form a salt. However alternative hole-trapping agents are still necessary in order to enlarge the number of possibilities to make more sensitive silver halide emulsions. Therefor a new type of platinum clusters is described hereinafter which can act as hole trapping agent.